Gold nanoparticles (GNPs) are useful as catalysts and for a wide variety of environmental, biomedical, and industrial chemical applications. To obtain GNPs, various wet chemical methods employing various polar and non-polar solvents have been used. The most common method is the reduction of tetralchloroauric acid (HAuCl4) by excess sodium borohydride (NaBH4) or sodium citrate in the presence of stabilizing/capping ligands such as citrate, thiolates, amines, phosphanes, carbonyls, dendrimers and surfactants. These methods have produced GNPs with sizes of 2-10 and 12-100 nm, respectively. A limitation of such methodology is that the GNPs produced by citrate methods beyond ˜50 nm are nonspherical and polydisperse. Hydroquinone has also recently been employed as a reducing agent to make relatively monodisperse GNPs with particle size up to 175 nm from GNP seeds synthesized by a traditional citrate method. However, the use of hydroquinone, a suspected carcinogen, in conjunction with traditional citrates methods leaves the product GNPs with trace amounts of organic solvents. This raises environmental concerns and also limits the biocompatibility and biomedical application of GNPs, for which avoidance of cellular toxicity is essential.
There is accordingly a need for a reliable, clean, and eco-friendly “green” chemical process for the synthesis of GNPs. Utilization of non-toxic chemicals and solvents for the synthesis and assembly of nanoparticles is a most important aspect of such a “green” nanoparticle synthesis process. Additionally, it is to be desired that such a process obviate any need for highly laborious size-sorting or seeding of any preformed crystals. It should be noted in this regard that nanotechnology requires the synthesis of nanomaterials of different sizes, shapes and controlled disparity for various life science related applications. In particular, the synthesis and subsequent linkage of GNPs with various biological and chemical materials find a wide array of applications in gene transfer, bioprobes for cell and tissue analysis, catalysis, information storage, imaging and drug delivery. In this context, exploiting GNPs in the size scale of >50 nm is crucial for many biomedical applications including biomimetics of biological molecules (protein, DNA) and structures (viruses, bacteria).